1. Field of the Invention
The present invention relates to flexographic printing processes. More particularly, the invention relates to a process of mask printing whereby a desired image is first printed on an uncoated paper substrate which is thereafter entirely overcoated with ink.
2. Description of the Prior Art
Watermarking is a process of faintly marking a sheet of paper with a permanent indicia. Such indicia is formed into the paper web during the papermaking process. No inks, colorants or resins are used. Localized orientation of the paper fiber in the indicia pattern provides an opacity difference within the body of the paper sheet.
Traditional watermarking dates back to a time when correspondence stationery was hand formed in single sheets from small batch vessels of aqueous pulp slurry. A forming screen stretched across a framed opening included the watermark pattern as woven into the screen matrix. Since each watermark was a distinctive original and the pulping and forming such a complex process, paper sheets possessing the watermark carried an association of prestige and security. Counterfeiting a watermark was extremely difficult and rarely attempted.
In similar fashion, watermarks are formed in current, machine laid paper by the use of specialized equipment on the fourdrinier or forming table. In lieu of single sheets of watermarked paper, however, tons of watermarked paper web is produced for subsequent cutting and slitting. Simple economics, therefore, limit the availability of distinctive, individualized, watermarked paper to only the largest stationery users.
To avoid the adverse economics of a genuine, wetlaid, watermark on relatively small quantities of stationery and fine paper, the prior art developed a simulated watermark process whereby colorless solutions of solid resins are printed onto a paper web or sheet. Penetration of the paper by the resin solution alters the paper opacity to translucency. Although barely perceptable when forelighted; the simulated watermark pattern is sharp and distinct when illuminated by back light.
Independently of the paper watermarking art, well defined two-tone printing of stationery and other fine papers is a common accomplishment of the reproductive art. However, with two or more colors, some form of register control between serial color stations is normally required. Exceptions are represented by simple, single color images such as line drawings printed on a substrate of contrasting shade; for example, a black line drawing on white paper substrate.
Obviously other contrasting colors may be used in the same manner: for example, a light blue line pattern printed on a dark blue paper substrate. In this latter example, however, the pattern field, i.e. the paper substrate, is the same color on both, obverse and reverse sides. Should a precisely aligned mirror image of the pattern be desired on the substrate reverse side, controlled register equipment is a practical necessity.
Considering a permutation of the light blue line pattern printed on a dark blue field, the printer may, at a first color station, coat the obverse side of a white paper substrate with dark blue ink and at a second color station, print the light blue pattern. By this process, the substrate reverse side remains white and devoid of the obverse side pattern.
It is an object of the invention, therefore, to teach an economical process for tinting or a coloring a paper substrate in conjunction with the application of a sharply distinctive line pattern or indicia in a contrasting shade of the same basic color used for the substrate tint.
Another object of the present invention is to provide a process for two-tone image printing using but one ink color.
Another object of the present invention is to teach a two-sided, show-through printing process. On a paper substrate obverse face side, a negative type two-tone image is shown wherein the optical density of the image field is relatively greater than that of the image lines. On the substrate reverse face, the relative optical densities are exchanged for a positive image relationship.
Another object of the invention is to provide a flexographic process for reproducing a pattern printed on the obverse face of paper substrate and simultaneously producing the mirror image of the pattern on the reverse face.